The present invention relates generally to a control for a hydraulic fluid distribution system utilized in an automotive vehicle lift rack having a pair of supporting runways each of which are elevated by at least one associated fluid actuated ram supplied from a common fluid reservoir, and more specifically, to a hydraulic fluid control circuit capable of compensating for uneven hydraulic fluid flow rates to and from the fluid actuated rams to maintain the runways in a level configuration under offset loading conditions at all times.
Traditionally, automotive repair shops and garages employ post-style hydraulic lifts having large hydraulic rams located below the floor of the garage to lift a pair of runways upon which a vehicle undergoing service is parked. These systems require excavations below the floor of the repair shop for the installation of the hydraulic ram or post, as well as for a hydraulic fluid reservoir and the associated plumbing. Due to increased regulations by the U.S. Environmental Protection Agency relating to the storage of potentially toxic fluids such as hydraulic fluid below ground, the trend in repair shops has been to utilize ground-level lift systems which do not require any below ground excavation or fluid storage.
Ground level lift systems, such as shown in U.S. Pat. No. 5,199,686 for "Non-Continuous Base Ground Level Automotive Lift System" and U.S. Pat. No. 5,096,159 for "Automotive Lift System", both to Fletcher are examples of parallelogram-style ground level automotive lifts. A comparable design is seen in U.S. Pat. No. 5,102,898 for "Control System for Vehicle Lift Racks" to Tsymberov. In the '686 and '898 systems, the pair of runways upon which the automotive vehicle undergoing service is parked are supported in an elevated position by separate hydraulic fluid rams or lifting elements. These rams are pressurized from a common hydraulic fluid circuit connected to an above-ground fluid reservoir. Several factors must be taken into consideration when designing and utilizing parallelogram lifts such as these. For example, there is a critical need to maintain each of the lift runways in a substantially parallel configuration at all times, despite the occurrence of uneven or offset loading conditions, as well as the need to maintain substantially the same fluid flow to each of the supporting hydraulic fluid rams during the raising or lowering of the lift runways.
As is shown in the '898 Tsymberov patent an even fluid distribution between the two or more hydraulic fluid rams can be achieved to some degree through the simple use of a flow divider/combiner valve, however, this is generally an inaccurate method of ensuring an even fluid distribution during offset loading conditions. The '686 Fletcher patent discloses the use of a complex arrangement of hydraulic control circuits and flow dividers utilized to coordinate the raising and lowering of the adjacent runways of a parallelogram lift, and to compensate for uneven flow rates of hydraulic fluid to each of the hydraulic rams. Specifically, the '686 patent employs a system control valve and a proportioning valve to control hydraulic fluid flow into and out of each supporting hydraulic fluid ram through both upper and lower ports. These circuits in the '686 patent utilize the arrangement of flow dividers and combination valves to either withdraw hydraulic fluid from a hydraulic cylinder which is elevating faster than another, or to withdraw additional hydraulic fluid from a hydraulic cylinder which is descending slower than another. In addition to incorporating a number of expensive components, these hydraulic fluid circuits often require lengthy calibration procedures to ensure that they are capable of maintaining a pair of runways in a substantially parallel configuration throughout the vertical operational range of the lift, even when loaded with an offset weight distribution.
Accordingly, there is a need to improve the design of the hydraulic fluid control circuits associated with these increasingly popular ground level lift systems and other lift systems having two or more independent lifting elements such that the circuits are inexpensive to manufacture, do not require extensive calibration and testing prior to operational installation, and are capable of maintaining the runways or lifting elements of the lift in a substantially parallel configuration throughout a vertical lift range despite severe offset loading conditions during the raising and lowering cycles.